Intro to seismic 2
- 1. RECAP LAST WEEK • Seismic reflection method – Acoustic based method which provides high resolution images of the sub- surface – Data recorded as shot gathers – many receivers for each source – Arrivals in data are identified from their character and moveout – Once identified their travel times can be used to determine depth to features in the sub-surface (within the errors of the input parameters) PETR8510
- 2. ZERO OFFSET DATA • Shot gathers – Numerous receivers, one source TWT – Data are recorded at ‘finite offset’ • Zero offset – Equivalent of one source, one receiver at each location, but in lots of places TWT – Much easier to interpret PETR8510
- 3. STACKING • Acquisition involves creating a series of shot gathers – Move-up rate – source vs receiver spacing PETR8510
- 4. STACKING • Acquisition involves an inherent data redundancy – Data recorded as shot gathers are sorted in to common mid point (CMP) gathers PETR8510
- 5. STACKING • CMP gathers are corrected for normal moveout (NMO) – This creates equivalent zero offset traces • Zero offset traces are plotted so centred on the equivalent source- receiver location Location TWT PETR8510
- 6. STACKING • NMO corrections in practice – Function of reflector depth, average velocity and source-receiver offset – Only offset is known • Resort to trial and errors and statistics PETR8510
- 7. STACKING • The zero offset section is not a depth section and its representation of sub- surface geometry is flawed – It needs to be ‘migrated’ • Post-stack migration only here PETR8510
- 8. MIGRATION • A reflector is the equivalent of a series of closely spaced diffractors – Migration = series of diffractor response corrections PETR8510
- 9. MIGRATION • Diffraction migration – Sum along diffraction travel time curve PETR8510
- 10. MIGRATION • Time migration – Based on a series of 1D velocity variations • Depth migration – Based on 2D velocity function Time Depth PETR8510
- 11. MIGRATION • Post-stack migration works provided dips are shallow • If not require pre-stack migration – More expensive – Based on ellipse rather than circle (in 2D) PETR8510
- 12. MIGRATION • Places features in their correct relative positions • Requires perfect knowledge of the sub-surface velocity field • Many methods • Pre- or post-stack – Depends on geological complexity – dips, lateral velocity variation • Computational intensive (expensive) PETR8510
- 13. SUMMARY • Stacking and migration – Improve SNR – Place features in ‘correct’ relative positions – Depth conversion difficult PETR8510
- 14. NEXT TIME • Well logging – measurements in a drill hole – Acoustic and density logging and synthetic seismograms • Other kinds of logging – Radioactivity (gamma), electrical (SP, resistivity) PETR8510